This invention relates to a DC chopper device, and more particularly to a transistor chopper device suitable for controlling a DC motor.
A DC chopper device is commonly employed in a system in which a DC power source such as a battery is used for the on-off control of a load. For example, in an electric vehicle, a DC chopper device is generally used for controlling a DC traction motor which drives the electric vehicle. There are a variety of types of DC chopper devices including the thyristor type and the transistor type.
These chopper devices are required to control a large electric power when used in, for example, electric vehicles, and therefore, elements constituting the chopper devices are also required to have a large capacity. Elements of large capacity are expensive compared with those of small capacity since such large-capacity elements are not mass-produced. Especially, the chopper device of the thyristor type is quite expensive since the chopper circuit has a complicated structure due to the necessity for incorporation of the commutation circuit.
Although transistor elements of large capacity are now available in the market, they are still more expensive than thyristors due to the low mass production rate. Thus, the high cost of semiconductor chopper elements of large capacity results inevitably in expensive chopper devices for use in electric vehicles. Efforts have heretofore been made to reduce the cost of DC chopper devices for use in electric vehicles, and such inexpensive DC chopper devices are disclosed in publications including U.S. Pat. Nos. 3,517,292, 3,569,742, 3,699,358 and 3,803,471, in which several or several tens of inexpensive power transistors of small capacity mass-produced for general application purposes are connected in parallel with each other to constitute chopper devices.
FIG. 1 shows a speed control system conventionally employed for controlling the speed of an electric vehicle by a DC chopper device comprising a plurality of parallel-connected power transistors. Referring to FIG. 1, the speed control system for controlling the speed of the electric vehicle comprises a battery B as a DC power source, a power supply on-off switch K, a series traction motor M having a series field winding FL, change-over switches F.sub.1, F.sub.2 and R.sub.1, R.sub.2 for changing over the polarity of the field winding FL to attain the change-over between the forward drive and the backward drive, a flywheel diode D, a transistor chopper TCH, a chopper control unit 10 controlling the transistor chopper TCH, and an accelerator unit 12 arranged for interlocking operation with the accelerator pedal of the electric vehicle for generating an electrical signal, in response to the depression of the accelerator pedal.
In operation, the power supply on-off switch K and the forward drive switches F.sub.1, F.sub.2 are turned on for the forward drive, and the output V.sub.OSC of the chopper control unit 10 is applied to the transistor chopper TCH for the on-off control of the transistor chopper TCH. The output V.sub.OSC of the chopper control unit 10 has a waveform as shown in FIG. 3. In the on state of the transistor chopper TCH, current is supplied from the battery B to the transistor chopper TCH by the route which is traced from the battery B.fwdarw.power supply switch K.fwdarw.forward drive switch F.sub.1 .fwdarw.series field winding FL.fwdarw.forward drive switch F.sub.2 .fwdarw.traction motor M to the transistor chopper TCH, thereby driving the traction motor M. In the off state of the transistor chopper TCH, flywheel current flows to the traction motor M through the flywheel diode D. The transistor chopper TCH repeats such an on-off operation. The ratio .alpha.=(T.sub.1 /T.sub.1 +T.sub.2), where T.sub.1 and T.sub.2 are the conducting and non-conducting periods of time of the transistor chopper TCH respectively as shown in FIG. 3, and is varied according to the instruction provided by the accelerator unit 12 so as to attain the desired speed control of the traction motor M.
The circuitry of a prior art transistor chopper TCH comprising such a plurality of parallel-connected power transistors will be described with reference to FIG. 2. Referring to FIG. 2, a drive transistor T.sub.0 is connected at its collector to a source of power supply voltage V.sub.cc in a control circuit through a collector resistor R.sub.C0, at its emitter to a common terminal E of the control circuit, and at its base to a control signal input terminal through a base resistor R.sub.10. The chopper control signal V.sub.OSC is applied to this control signal input terminal. A plurality of power transistors T.sub.1 to T.sub.5 which are driven by the drive transistor T.sub.0 are connected at their emitters to the common terminal E through balancing emitter resistors R.sub.E1 to R.sub.E5 respectively and at their bases in parallel with the collector of the drive transistor T.sub.0 through base resistors R.sub.11 to R.sub.15 respectively. The power transistors T.sub.1 to T.sub.5 are directly connected at their collectors to a terminal C connected to the traction motor M shown in FIG. 1, and the common terminal E, to which the emitters of the power transistors T.sub.1 to T.sub.5 are common-connected, is connected to the negative terminal of the battery B in FIG. 1 to constitute the chopper circuit.
In operation, the power transistors T.sub.1 to T.sub.5 are turned on in response to the application of the chopper control pulse V.sub.OSC, having the waveform shown in FIG. 4, to the base of the drive transistor T.sub.0. More precisely, when the chopper control pulse V.sub.OSC takes its low level as shown in FIG. 4, the drive transistor T.sub.0 is turned off, and base current is supplied to the power transistors T.sub.1 to T.sub.5 through the collector resistor R.sub.CO for the drive transistor T.sub.0 and through the base resistors R.sub.11 to R.sub.15 for the respective power transistors T.sub.1 to T.sub.5, with the result that collector currents I.sub.C1 to I.sub.C5 flow into the collectors of the respective power transistors T.sub.1 to T.sub.5. In the high level of the chopper control pulse V.sub.OSC, the drive transistor T.sub.0 is turned on to cut off all the power transistors T.sub.1 to T.sub.5.
FIG. 5 shows, by way of example, the waveforms of the collector currents I.sub.C1 and I.sub.C2 of the power transistors T.sub.1 and T.sub.2 among the power transistors T.sub.1 to T.sub.5 connected in parallel in the manner shown in FIG. 2. It will be seen from FIG. 5 that fluctuation of the turn-off characteristic of such power transistors results frequently in concentration of current flow on a specific power transistor in the transient state, and this specific power transistor may be damaged when the current value exceeds the current rating of the transistor. According to the prior art practice for dealing with this problem, power transistors having the same turn-off characteristic are selected for constituting the transistor chopper circuit. However, this method is impractical in that, because of the necessity for selection of power transistors having the same turn-off characteristic from among many transistors produced by mass production, a limited number of transistors only can be used for the purpose, resulting also in an increase in the cost of the transistors. According to another method proposed hitherto to deal with the above problem, power transistors having turn-off characteristics lying within an allowable range of fluctuation are selected to operate in a condition in which the operating current value is sufficiently lower than the maximum rating of each individual transistor, and the number of parallel-connected power transistors is increased to avoid objectionable concentration of current flow in a specific power transistor. However, this latter method requires a greater number of power transistors than the former method when the load current is the same, since the power transistors are used to operate with sufficient margins in their operating performance.